There is a bolt and wedge to secure the stem (gooseneck). Did you remove the bolt? If you did, you should screw it back in a few turns, and tap the end with a mallet. That will break free the wedge so you can remove the stem. If you did not, look down inside the stem. There may be a shoulder and you will see a 6mm hex bolt. Loosen it a few turns and again, tap it with a mallet to break free the wedge, and remove the stem. Then you can loosen the headset locknut and proceed with the bearing service.